1. Field of the Invention
The present invention relates to a C12A7 electride thin film fabrication method and a C12A7 electride thin film.
2. Description of the Related Art
Crystalline C12A7 has a representative composition expressed as 12CaO.7Al2O3 (hereinafter referred to as “C12A7”) and has a characteristic crystal structure including voids (cages) with diameters of about 0.4 nm that are linked three-dimensionally. The lattice framework forming the cages is positively charged and forms 12 cages per unit cell. One-sixth (⅙) of the cages are occupied by oxygen ions in order to satisfy an electrically neutral condition of the crystal. These oxygen ions are particularly referred to as free oxygen ions because they have properties chemically different from those of the other oxygen ions constituting the framework. Crystalline C12A7 is also denoted as [Ca24Al28O64]4+.2O2− (see F. M. Lea and C. H. Desch, The Chemistry of Cement and Concrete, 2nd ed., p. 52, Edward Arnold & Co., London, 1956).
Also, 12SrO.7Al2O3 (hereinafter referred to as “S12A7”) is known as an isomorphic compound of crystalline C12A7, and a mixed crystal compound of C12A7 and S12A7 containing Ca and Sr at a given mix ratio is also known (see O. Yamaguchi, A. Narai, K. Shimizu, J. Am. Ceram. Soc. 1986, 69, C36).
The inventors of the present invention, Hosono et al., have discovered that by subjecting crystalline C12A7 powder or a sintered body thereof to a heating process in a H2 atmosphere to cause the cages to contain H-ions and subsequently irradiating ultraviolet light thereon, the crystalline C12A7 powder or the sintered body may be arranged to contain electrons in the cages and be able to exhibit permanent conductive properties at room temperature (see WO 2005/000741). The electrons contained are loosely bound to the cages and can move freely in the crystals, and as such, crystalline C12A7 exhibits conductive properties.
Crystalline C12A7 having such conductive properties is referred to as a crystalline C12A7 electride. Because crystalline C12A7 electrides have a very low work function of about 2.4 eV, advantageous effects can be expected by applying crystalline C12A7 electrides to electron injection electrodes of organic EL (electroluminescence) elements and cold electron emission sources, or reducing agents used in chemical reactions, for example.
Generally, a crystalline C12A7 bulk is fabricated by performing a sintering process on crystalline C12A7 electride powder under a high-temperature reducing atmosphere (Patent Document 1). For example, the temperature of the sintering process may be about 1200° C.
Although the above conventional method may be effective for fabricating the crystalline C12A7 in bulk, the method is not suitable for fabricating a crystalline C12A7 electride thin film.
That is, in the case of fabricating a crystalline C12A7 electride thin film using a conventional method requiring a high temperature of about 1200° C., for example, materials that may be used as the substrate of the thin film may be limited to heat resistant materials. As a result, the combination of the types of materials that may be used for the thin film and the substrate may be very limited.
For example, a glass substrate is often used as a versatile substrate for various types of electrical devices and elements. However, the temperature limit for a glass substrate is about 700° C. at most. Accordingly, it would be difficult to form a crystalline C12A7 electride thin film on a glass substrate using a conventional fabrication method owing to the temperature limit of the glass substrate.
In light of the above, there is a growing demand for a technique for fabricating a C12A7 electride thin film under a low processing temperature to avoid or alleviate the above-described shortcomings.